A method and device of this kind are known from Yukinori Ochiai e.a., "Focused Ion Beam Technology", Solid State Technology/November 1987, pp 75-78.
The cited article describes a method where patterns are inscribed in a photoresist layer on a semiconductor substrate by means of a focused ion beam, or ions are implanted directly in the semiconductor substrate. To achieve this, an ion beam is generated in a charged particle beam system, for example by means of a liquid metal ion source, which beam is accelerated to some hundreds of keV by means of electrodes arranged along an optical axis. Using an electrostatic condensor lens, the ion beam is imaged in a focused manner in a mass-separation system in which mutually perpendicular electrostatic and magnetic fields transmit only ions of appropriate mass (ExB filter). An electrostatic stigmator corrects the ion beam for deviations in the cross-section which is circular in the ideal case. After having passed an objective lens which comprises a number of electrodes which are arranged so as to be spaced apart around the optical axis and which have a circular aperture, the ion beam is deflected across the semiconductor substrate by means of a deflection element. The diameter of the ion beam on the substrate then amounts to 0.1 .mu.m, and a beam current may amount to from 1 .mu.A to 100 .mu.A. The ion energy amounts to from 10 to 150 keV at the area of the substrate.
For shallow implantation of ions, directly below the surface of the irradiated object, it is desirable to implant ions having a low energy (less than 1 keV). A problem encountered in the case of such low energies consists in that for current densities of approximately 100 .mu.A the beam diameter increases substantially due to the effect of the radial Coulomb interaction between the ions; this interaction is large for the given current at a comparatively low speed of the ions (the space charge effect). A drawback of a large beam diameter consists in that for uniform implantation of a substrate the beam must be deflected as far as beyond the substrate. For a substrate having a diameter of, for example 10 cm, in the case of a beam diameter of 5 cm a surface which is approximately four times greater than the surface of the substrate must be scanned. As a result, the duration of implantation is unnecessarily long, thus impeding efficient production of large numbers of implanted substrates. Furthermore, in the case of prolonged implantation the risk of contamination of the substrate is comparatively high.